HOW THE DRAINFIELD WORKS

Serial drainfield distribution on slope

Note how riser pipes at end of distribution pipes cause effluent to flow to lower pipes only when upper pipes are full.

The drainfield provides both disposal and treat­ment of the septic tank effluent. Effluent flows from the septic tank to the drainfield through a watertight pipe and is then distributed within the drainfield trenches through perforated pipes in the gravel. The effluent flows through the gravel filling and then seeps (infiltrates) into the soil beneath and beside the trench. Here the main purification of the wastewater takes place through filtration and biological activity as it infiltrates through the biological mat (seep. 19) on the side- walls and at the bottom of the trench and then percolates through unsaturated soil. (However, in seasoned fields, there is practically no perco­lation through the trench bottom.) The purified liquid then eventually evaporates, is taken up by plants, or percolates into the groundwater.

MOVEMENT OF EFFLUENT

Once the effluent leaves the septic tank, it flows:

  • to the trenches through a watertight distribution system
  • into the trench gravel through perforated pipes
  • into the soil through the infiltrative surfaces (gravel-soil interface)
  • through unsaturated (or saturated) soil beside and beneath the trench

Finally, it is either taken up by plants or perco­lates to groundwater.

TREATMENT OF EFFLUENT

Effluent gets treated in the drainfield:

  • as it infiltrates into the soil: the biomat is the tool. (See adjacent column.)
  • as it percolates through the soil
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